Compact Eccentric Radial Piston Hydraulic Machine

ABSTRACT

A high efficiency diametrically compact, radial oriented piston hydraulic machine includes a cylinder block with a plurality of cylinders coupled to a first port by a first valve and to a second port by a second valve. A drive shaft with an eccentric cam, is rotatably received in the cylinder block and a cam bearing extend around the eccentric cam. A separate piston is slideably received in each cylinder. A piston rod is coupled at one end to the piston and a curved shoe at the other end abuts the cam bearing. The curved shoe distributes force from the piston rod onto a relatively large area of the cam bearing and a retaining ring holds each shoe against the cam bearing. The cylinder block has opposing ends with a side surface there between through which every cylinder opens. A band engages the side surface closing the openings of the cylinders.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent Application No. 61/181,117 filed on May 26, 2009.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to hydraulic machines, such as pumps and hydraulic motors, and more specifically to such machines that have pistons that move radially against an eccentric shaft.

2. Description of the Related Art

A common type of radial piston pump comprises a body with a plurality of cylinders radially disposed around a drive shaft for accommodating pistons. A piston is slideably received within each cylinder, thereby defining a chamber at the interior of the cylinder. The shaft has an eccentric cam and the pistons are biased by springs to ride against that cam with line contact between the piston and cam. The line contact between the piston and the eccentric cam of the conventional pump limits the load bearing capability of the device and puts a force moment on the piston to bore interface. An inlet port supplies fluid to an inlet passage that is coupled through a separate inlet check valve to each cylinder chamber. A set of outlet check valves couples the cylinder chambers to an outlet passage that leads to an outlet port of the pump.

As the drive shaft is rotated by an external motor or engine, the eccentric cam causes the pistons to slide cyclically in and out of the cylinders, thereby reducing and expanding the volume of the respective cylinder chamber. During an intake phase of each piston cycle, when a given cylinder chamber volume is expanding, the inlet check valve opens allowing fluid to be drawn from the inlet passage into the cylinder chamber. During the subsequent exhaust phase of each piston cycle, when the volume of the cylinder chamber is reducing, fluid is expelled under pressure through the outlet check valve though the outlet port. The fluid intake and exhaust phases occur repeatedly during every rotation of the eccentric cam. At any point in time, some of the radially disposed cylinders are in the intake phase and other cylinders are in the exhaust phase. When the conventional check valve pump is at full stroke the noise level is relatively low as there is no high pressure metering noise as found in valve plate or pintal metering of axial and external eccentric radial piston pumps and motors.

Conventional radial piston pumps typically are relatively large in diameter in order to accommodate the biasing springs and plugs that close an outer end of each cylinder. In many installations the amount of space for the pump is limited, thus it is desirable to reduce the size of the pump. More specifically, many times the pump is mounted along side an engine or transmission and the radial space is limited preventing the installation of typical radial piston pumps.

Another issue related to radial piston pumps is that as the drive shaft rotates, a moment of force is exerted on the rod of each piston. That moment results in side forces that push the piston against the wall of the cylinder in which the piston slides. Such side forces impede the sliding motion of the piston and thus are undesirable.

An additional efficiency issue with internal eccentric radial piston pumps results from a requirement that the case be full of fluid for either displacement control or for lubrication of sliding friction surfaces. With a full crankcase, the rotating eccentric cam encounters significant windage loss that lowers the efficiency of the device. Another benefit is that this design with no sliding friction elements, except the lightly loaded piston to bore motion, can be used in low lubricity oils and at reduced pressures even with liquids such as water. This invention solves the problems of prior pumps by providing of radial compactness, enhanced power density and improved efficiency as follows.

SUMMARY OF THE INVENTION

The novel hydraulic machine includes a cylinder block having two end surfaces with a side surface there between. A first port and a second port are formed in the cylinder block for making hydraulic connections thereto. A plurality of cylinders is disposed radially in the cylinder block and each cylinder has an opening through the side surface. A closing band engages the exterior surface and closes the openings of the plurality of cylinders. A separate piston assembly is slideably received in each of the plurality of cylinders. A drive shaft is rotatably received in the cylinder block and has an eccentric cam for driving the plurality of cylinders within the plurality of cylinders.

One aspect of the hydraulic machine provides at least one valve associated with each cylinder to control fluid flow between the cylinder and each of the first and second ports. In one embodiment, a plurality of first bores is formed in one of the surfaces, and a plurality of second bores is formed in the same or the other end surface. A plurality of first valves is located in the first bores, selectively providing a fluid path between the first port and one of the plurality of cylinders. A plurality of second valves is located in the second bores and selectively provides another fluid path between the second port and one of the plurality of cylinders. In one version, each of the first and second valves is passive. In another version, each of the first and second valves is electrically operated and preferably is a two-position, two-way valve.

In another embodiment, a plurality of bores is formed in the first end surface. A plurality of three-way valves is located in the first bores selectively providing a fluid path between one of the plurality of cylinders and the first and second ports. In a preferred version, each three-way valve is a three-position valve that is electrically operated.

Another aspect of the present hydraulic machine is that each piston assembly comprises a piston to which a piston rod is connected. The piston rod includes a stem with a curved shoe that has a surface through which force is applied to the eccentric cam. The surface of the piston rod shoe has an area that is greater than the largest cross sectional area of the stem. The shoe distributes the forces from the piston rod onto a larger area of the eccentric cam, thereby enabling higher load bearing. A retaining ring extends around the drive shaft and engages the curved shoe of every piston rod, thereby holding each piston rod toward the eccentric cam. This retaining ring eliminates a need for springs used in previous machines to bias the pistons or the piston rod against the drive shaft cam.

Also described are several alternative arrangements for coupling the piston rod to the piston to reduce lateral forces between the piston and the wall of the cylinder. Such lateral forces tend to impede the sliding of the piston in the cylinder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an open loop hydraulic system that incorporates a radial piston hydraulic machine according to the present invention;

FIG. 2 is a radial cross section showing the arrangement of the cylinders and pistons in the hydraulic machine;

FIG. 3 is an axial cross section through the radial piston hydraulic machine along line 3-3 in FIG. 2;

FIG. 4 is a partial radial cross section showing a second arrangement of a piston and piston rod;

FIG. 5 is a partial radial cross section illustrating a third arrangement of a piston and piston rod;

FIG. 6 is a partial radial cross section of a variation of the embodiment in FIG. 3 in which the inlet and outlet check valves for each cylinder are in a common bore in the cylinder block of the hydraulic machine;

FIG. 7 is a schematic diagram of a version of the hydraulic machine that may operate a motor in a bidirectional manner;

FIG. 8 is a schematic diagram of a hydraulic system that incorporates a version of a hydraulic machine according to the present invention that can be operated as both a pump and a motor;

FIG. 9 is an axial cross section through the radial piston hydraulic machine that utilizes a pair of two-position, two-way spool valves to control fluid flow to and from each cylinder; providing closed loop capability and enabling the hydraulic machine to function as both pump and motor; and

FIG. 10 is an axial cross section through the radial piston hydraulic machine that utilizes a single three-position, three-way spool valve to control fluid flow to and from each cylinder.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, an open loop hydraulic system 10 has a prime mover 12, such as an internal combustion engine or an electric motor, that is coupled by a shaft to drive a hydraulic machine 14 to function as a pump. The hydraulic machine 14 can be configured as a fixed displacement pump to draw fluid in from the first conduit 15 and force the fluid under pressure into the second conduit 16, thereby driving a hydraulic motor 18 in one direction. The hydraulic motor 18 rotates one or more wheels 20 of a vehicle, for example.

The same design of the hydraulic machine also can be used as a hydraulic motor, such as hydraulic motor 18. Here the hydraulic machine receives pressurized fluid at one port and converts that fluid power into mechanical energy that is applied to a shaft connected to wheels 20.

Therefore, the apparatus described herein is generically referred to as a “hydraulic machine” since can be configured to function as both a pump and a hydraulic motor depending upon how and where is used in a hydraulic system. In some situations the same hydraulic machine may operate as both a pump and a motor at different times depending upon whether the machine is driving the load, such as wheels 20, or is being driven by the load, such as when the vehicle coasts to a stop.

With reference to FIGS. 2 and 3, the hydraulic machine 14 has a cylinder block 30 with exterior first and second end surfaces 21 and 22 between which a circular exterior side surface 38 extends. The cylinder block 30 has an inlet port 28 and an outlet port 29 to which conduits 15 and 16 respectively connect. The inlet and outlet ports 28 and 29 connect to annular inlet and outlet passages 31 and 32, respectively, that extend in circles through the cylinder block around a central shaft bore 41 that extends through the cylinder block 30. Three cylinders 36 extend radially outward from and are oriented at 120 degree increments around the central shaft bore 41. Although the illustrated embodiment has three cylinders 36, in practice, the hydraulic machine 14 may have a greater number of cylinders to reduce torque, flow and pressure ripples machine operation. Each cylinder 36 comprises a tubular sleeve 39 that is inserted into a bore in the cylinder block 30. Although the sleeve 39 is beneficial in enabling a reduction in the diameter of the hydraulic machine 14 as will be described, the sleeve can be eliminated by using a material for the cylinder block that can be machined to form the cylinders. Each cylinder 36 has an opening through a side surface 38 of the cylinder block 30. A sealing cup 24 with an O-ring is placed inside each opening and a band-shaped closing ring 35 extends around the side surface 38 tightly closing each of the cylinder openings. The continuous closing ring 35 eliminates the relatively long plugs projecting outward from each cylinder in conventional pump designs and thereby reduces the overall diameter of the hydraulic machine 14.

With particular reference to FIG. 3, a plurality of first bores 26 extends into the first end surface 21 of the cylinder block 30 and each bore opens into both the inlet passage 31 and a respective one of the cylinders 36. A separate inlet check valve 33 is located each of those first bores 26. The inlet check valve 33 opens when the pressure within the inlet passage 31 is greater than the pressure within the cylinder chamber 37, as occurs during the intake phase of the pumping cycle. A plurality of second bores 27 extends into the second end surface 22 of the cylinder block 30 with each second bore opening into both the inlet passage 31 and a respective one of the cylinders 36. A separate outlet check valve 34 is located each of those second bores 27. The outlet check valve 34 opens when pressure within the cylinder chamber 37 is greater than the pressure within the outlet passage 32 as typically occurs during the exhaust phase of the pumping cycle. The inlet and outlet check valves 33 and 34 are structurally and functionally the same as similar types of check valves employed in conventional pumps. It should be understood that the inlet and outlet passages 31 and 32 communicate with all the piston cylinders in the pump and am identical pair of check valves is provided for each cylinder. Each of the inlet and outlet check valves 33 and 34 is passive meaning that it operates in response to pressure exerted thereon and not by an electrical actuator, such as a solenoid.

The tubular sleeve 39 that partially forms the cylinder 36 enables the inlet and outlet check valves 33 and 34 to be placed closer to the longitudinal axis 25 of the drive shaft 40. Note that the inlet and outlet check valves 33 and 34 are within the closed curved perimeter defined by the exterior side surface 38 of the cylinder block 30. In prior configurations the valves had to be outward from the top dead center position of the piston in order to receive the fluid forced out of the cylinder chamber 37. As shown in FIG. 3, the tubular sleeve 39 extends partially over the opening between the cylinder chamber 37 and the bores in which the check valves 33 and 34 are located thereby extending the cylinder bore farther into the cylinder chamber 37.

Referring again to both to FIGS. 2 and 3, a drive shaft 40 extends through the shaft bore 41 and is rotatable therein being supported by a pair of bearings 42. The central section of the drive shaft 40 within the cylinder block 30 has an eccentric cam 44. The cam 44 has a circular outer surface, the center line of which is offset from the axis 25 of the remainder 43 of the drive shaft 40. As a consequence, as the drive shaft 40 rotates within the cylinder block 30, the cam 44 rotates in an eccentric manner about the axis 25 of the drive shaft. As specifically shown in FIG. 2, a cam bearing 46 extends around the cam 44 of the drive shaft 40. The cam bearing 46 has an inner race 47 that is pressed onto the outer circumferential surface of the cam and an outer race 48. A plurality of rollers 49 are located between the inner and outer races 47 and 48. In a preferred embodiment, the inner race 47 is eliminated by proper heat treating and machining of the shaft 40 eccentric cam 44 to function as that race. The cam bearing 46 improves the efficiency of the hydraulic machine over conventional pumps and motors that use a sliding journal bearing for this function. Although a cam bearing 46 with cylindrical rollers is depicted, a bearing with spherical or other types rollers may be used.

A piston assembly 51 is slideably received within each of the cylinders 36. Each piston assembly 51 comprises a piston 52 and a piston rod 54. The piston rod 54 extends between the piston 52 and the cam bearing 46. The piston rod 54 has a curved shoe 56 which abuts the outer race 48 of the cam bearing 46. The shoe 56 is wider than the shaft of the piston rod creating a flange portion. A pair of annular retaining rings 58 extend around the cam 44 engaging the flange portion of each piston rod shoe 56, thereby holding the piston rod 54 against the cam bearing 46, which is particularly beneficial during the intake stroke portion of a pumping cycle. The curved shoe 56 evenly distributes the piston load onto the outer race 48 of the cam bearing 46 and also distributes the local load onto the rollers 49 of that bearing. The shoe 56 distributes the load over a wide area which prolongs bearing life and contributes to the compactness and pressure rating capability of the overall machine design as compared to a conventional pump or motor. As will be described in greater detail, as the drive shaft 40 and cam 44 rotate within the cylinder block 30, the outer race 48 of the cam bearing 46 remains relatively stationary. The outer race 48 rotates at a very slow rate in comparison to the speed of the drive shaft. Therefore, there is little relative motion between each piston shoe 56 and the cam bearing's outer race 48.

The piston 52 is cup-shaped having an interior cavity 53 which opens toward the drive shaft 40. An end of the piston rod 54 is received within that interior cavity 53 and has a partially spherical head 60 that fits into a mating partially spherical depression 62 in the piston 52. The head of the piston 52 may have an aperture 50 there through to convey hydraulic fluid from the cylinder chamber 37 to lubricate the interface between a spherical head 60 and the piston 52. The piston rod 54 is held against the piston 52 by an open single bushing or a split bushing\55 and a snap ring 57 that rests in an interior groove in the piston's interior cavity 53. As the piston rod 54 follows the eccentric motion of the cam 44 and the piston 52 in turn follows by sliding within the cylinder 36. The bushing and snap ring arrangement allows the spherical head 60 of the piston rod to pivot with respect to the piston 52 when a rotational moment is imposed onto the piston rod 54 by rotation of the cam 44. Because of that pivoting, the rotational moment is not transferred into the piston 52, thereby minimizing the lateral force between the piston and the wall of the cylinder 36.

FIG. 4 illustrates an alternative mechanism 70 for securing the piston rod 54 to the interior of the piston 52. As with the previous embodiment, the partially spherical head 60 of the piston rod 54 fits into the mating depression 62 in the end surface 61 of the piston's interior cavity 53. An annular groove 72 extends around the interior cavity 53 spaced from the end surface 61. A conically shaped spring 74 has a larger end which rests within the annular groove 72 and a smaller end that extends around the piston rod and engages the spherical head 60 of the piston rod 54. The conical spring 74 biases the spherical head 60 into engagement with the piston 52, thereby holding those two components abutting each other. Therefore, when the rotation of the cam 44 pulls the piston rod 54 downward in the orientation of FIG. 4, the spring 74 pulls the piston 52 downward along with that piston rod.

FIG. 5 illustrates another alternative embodiment in which the piston and piston rod are constructed as a single piece of metal. Here the piston 80 has a circular cross section in a plane that is transverse to the plane of the drawing thus fitting within a circular cylinder 36. However, the annular surface 82 which engages the wall of cylinder 36 has a spherical contour so as to be able to pivot with respect to the axis of the cylinder 36 and still maintain a tight engagement with the cylinder. This pivoting capability accommodates a slight wobble of the piston rod as the cam 44 rotates. The annular surface 82 has a seal 84 to ensure a fluid tight engagement with the cylinder wall.

In this embodiment, the piston rod 86 is integral with the piston 80 and extends downward to a curved shoe 88 that abuts the outer surface of the outer race 48 of the cam bearing 46. At least one annular retaining ring 58 holds the piston rod shoe 88 against the outer race 48. Therefore, as the cam 44 rotates eccentrically within the cylinder block 30, the piston rod 86 and the integral piston 80 are pushed and pulled along the cylinder 36. By accommodating a slight wobble of the piston rod, the lateral forces, or side load, between the piston and the cylinder is minimized.

Returning to FIG. 3, the drive shaft 40 includes an internal lubrication passage 64 extending from one end to the outer surface of the cam 44. The lubrication passage 45 has a single opening in that outer surface at the center of the eccentric apex of the cam to feed fluid into the cam bearing 46. The other end of the lubrication passage 64 opens into a chamber 66 at the end of the drive shaft 40 and that chamber receives relatively low pressure fluid through a feeder passage 68 from the inlet passage 31. As the drive shaft 40 rotates, centrifugal force expels fluid from the lubrication passage 64 into the cam bearing 46. This action draws additional fluid into the lubrication passage 64 from the chamber 66, thereby providing a pumping function for fluid that lubricates the cam bearing 46. If the cam bearing 46 has an inner race 47, that inner race has apertures that convey the lubricating fluid to the rollers 49. The outer race 48 also has through holes to lubricate the shoes 56 of the piston rods 54, thereby providing splash lubrication and eliminating a need to have the central shaft bore 41 filled with fluid. Not having the crankcase filled with fluid reduces windage drag on the eccentric cam 44 and improves efficiency of the hydraulic machine. Additional lubricating passages 59 are provided to convey fluid from the shaft bore 41 to the tapered bearings 42 for the drive shaft 40. The fluid used for lubrication exits the central shaft bore 41 through a standard drain port 69 from which the fluid is conveyed to a tank for the hydraulic system.

The cylinder block 30 in FIG. 3 has separate bores 26 and 27 projecting inward from the opposing first and second end surfaces 21 and 22 within which the inlet and outlet check valves are received. FIG. 6 shows an alternative design in which a single bore 90 projects from one of those end surfaces In the illustrated example, this single bore 90 extends from an opening 91 in the first end surface 21 through the inlet passage 31 and the associated cylinder 36 terminating on the other side at an opening into the outlet passage 32. Nevertheless the single bore could extend from the second end surface 22 to the inlet passage 31.

The outlet check valve 34 comprises a first valve element 92 with a stem 93 that extends into a blind guide aperture 94 in the cylinder block 30. An annular first valve seat 95 is press fitted into the bore 90 between the outlet passage 32 and the cylinder 36. A coil compression spring 96 biases the first valve element 92 into engagement with the first valve seat 95.

On the opposite side of the cylinder 36, the inlet valve 33 is press fitted into the bore 90 between the inlet passage 31 and the cylinder. The inlet valve 33 comprises a second valve element 97 biases by a second spring 99 into engagement with a second valve seat 98. Both the inlet and outlet check valves 33 and 34 are inserted into the bore 90 through the opening 91 in the second end surface 22.

The hydraulic machine 14 illustrated in FIGS. 2 and 3 can be employed to drive the motor 18 in either rotational direction. In this implementation show in FIG. 7, a two-position, three-way directional valve 130 couples the inlet and outlet ports 28 and 29 of the hydraulic machine 14 to the first and second conduits 15 and 16 that connect to the motor 18. Depending on the position of the directional valve 130, the inlet passage 31 is connected to one of the first and second conduits 15 and 16, and the outlet passage 32 is connected to the other of those conduits. The controller 122 operates the electrohydraulic directional valve 130.

FIG. 8 depicts another hydraulic system 100 that employs a bidirectional embodiment of a hydraulic machine 114 that is driven by a prime mover 112. The hydraulic machine 114 can be configured as a fixed displacement, bidirectional pump that is controlled to force fluid in either direction through conduits 115 and 116 connected to a bidirectional hydraulic motor 118 that rotates a wheel 120 of a vehicle, for example. The hydraulic machine 114 is dynamically configured to draw fluid in from either conduit 115 or 116 and force the fluid under pressure into the other conduit, thereby driving the motor 118 in either direction. The operation of the hydraulic machine 114, as will be described, is governed by a controller 122 that receives commands from an operator input device 124.

The details of one version of a bidirectional hydraulic machine that may be used in hydraulic system 100 are illustrated in FIG. 9. This hydraulic machine 200 has a basic construction similar to that of the machine 14 in FIGS. 2 and 3 and thus identical components of both machines have been assigned the same reference numerals. Specifically a plurality of cylinders extend radially and open through a circular exterior side surface 207 of the cylinder block 201. A significant difference is that the hydraulic machine 200 utilizes electrohydraulic two-position, two-way spool valves 202 and 204 in place of passive check valves 33 and 34 in the previous machine.

Each two-position, two-way first valve 202 extends into a separate first bore 209 in a first end surface 203 of the cylinder block 201. The first valve 202 has a spool 206 that controls the flow of fluid through the first bore 209 between the cylinder chamber 37 and an annular first passage 221 in fluid communication with a first port 224. The first valve 202 has a first solenoid 208 that moves the spool 206 between open and closed positions in a manner that replicates the action of the first check valve 33 in the machine embodiment of FIG. 3. The first solenoid 208 is driven by the microcomputer based controller 122 in response to a signal from a sensor 210 that indicates the position of the drive shaft 40.

Each two-position, two-way second valve 204 extends into a separate second bore 211 in a second end surface 205 of the cylinder block 201. The second valve 204 selectively opens and closes a path through the second bore 211 between the cylinder chamber 37 and an annular second passage 222 in fluid communication with a second port 226. This second valve 204 has a second solenoid 214 that moves the second spool 212 in the second bore 211 to control into the open and closed positions in a manner that replicates the action of the second check valve 34 in FIG. 3. The controller 122 also operates that second solenoid 214. This operation of the first and second valves 202 and 204 pumps fluid from the first port 224 to the second port 226. Note that the spools 206 and 212 of the first and second valves 202 and 204 are located between drive shaft axis 25 the outward most position of the head of the piston 52 during each cycle. That outward most position being depicted in FIG. 9. The spools 206 and 212 of the first and second valves 202 and 204 are oriented parallel to the axis 25 of the drive shaft 40, that is the spools move along a parallel axis. This orientation further reduces the overall diameter of the hydraulic machine 200.

The first and second bores 209 and 211 are generally coplanar with their associated cylinder 36. For more compactness in the longitudinal axial dimension, the first and second bores 209 and 211 and thus the first and second electrohydraulic valves electrohydraulic valves 202 and 204 therein can be offset and be located between two adjacent cylinders.

The pair of electrohydraulic valves 202 and 204 are operated to generally replicate the action of the two check valves 33 and 34 in the embodiment of FIG. 3. Nevertheless, the first and second electrohydraulic valves 202 and 204 are both closed at the end of the pumping cycle, the transition between the exhaust and intake phases, which results in the force of cylinder pressure decompression is conveyed onto the drive shaft 40. In prior pumps, the pressure remaining in the cylinder at the end of the exhaust phase was expelled into the inlet passage 31 when the inlet valve opened. This had two adverse effects, firstly back pressure was fed into the inlet passage affecting the aspiration of other cylinders in the intake phase and secondly the decompression cause significant operational noise. These adverse effects are diminished by the present technique in which the force of cylinder pressure decompression is conveyed onto the drive shaft 40.

In addition to replicating the action of the two check valves 33 and 34 in the embodiment of FIG. 3, the pair of electrohydraulic valves 202 and 204 be operated in a reverse manner so that the flow of fluid through the pump is from the second port 226 to the first port 224, thereby reversing the inlet and outlet ports. This reverse operation enables the hydraulic machine 200 to pump fluid by bidirectionally. With bidirectional operation, a separate third port 228 for supplying lubrication fluid is required, as either port 224 or 226 will have relatively high pressure when serving as the outlet port.

This selective bidirectional valve control also enables the hydraulic machine 200 to function as a motor. For example, when the vehicle in FIG. 7 is coasting to a stop, the wheel 120 drives the motor 118 forcing fluid backwards to the hydraulic machine 114. By configuring the hydraulic machine 114 to act as a motor, the energy in that fluid can be recovered and to supplement the prime mover 112. To configure the hydraulic machine 200 in FIG. 9 to act as a motor, whichever one of the first or second valve 202 or 204 is associate with the port 224 or 226 that receives the pressurized fluid from the motor 118, is opened by the controller 122 during the intake phase of the piston cycle. This allows the pressurized fluid to enter the cylinder chamber 37 and exert force on the piston 52 to rotationally drive the eccentric cam 44 and thus the entire drive shaft 40. When the piston 52 reaches bottom dead center and transitions into the exhaust phase, the previously opened valve 202 or 204 closes and the other one of those valve opens. Thus during the exhaust phase, the fluid is forced out of the cylinder chamber 37 and through the other port 226 or 224 of the hydraulic machine 200. This cycle repeats as long as the motor 118 acts as a pump.

The present hydraulic machine 200 is more efficient and quieter than conventional valve plate or pintal pump/motor designs. In the present hydraulic machine, both valves 202 and 204 can be closed to allow the cylinder chamber 37 to decompress, while putting energy back into the drive shaft 40. This action improves efficiency and such decompression eliminates high pressure drop metering and corresponding noise generation experienced with valve plate or pintal fixed geometry metering hydraulic machines.

Yet another hydraulic machine 300 is depicted in FIG. 10 and is constructed similar to the first hydraulic machine 14 in FIGS. 2 and 3. Identical components in both machines have been assigned the same reference numerals. A major distinction is that this hydraulic machine utilizes a single electrohydraulic three-position, three-way valve 302 in place of the inlet and outlet check valves 33 and 34 in the previously described machine. Specifically, the cylinder block 301 has a plurality of bores 305 extending from an end surface 303 parallel to the longitudinal axis 25 of the drive shaft 40 which contributes to the compact structure of the hydraulic machine 300. Each of those bores 305 communicates with first and second annular passages 321 and 322 and with one of the cylinders 36. The first annular passage 321 opens into a first port 324 and the second annular passage 322 opens into a second port 326. A separate third port 328 for supplying lubrication fluid is provided.

The single control valve 302 for each cylinder 36 extends into the associated bore 305 and has a spool 306 that is moved into different positions by a solenoid 308 which is operated by the controller 122 in response to signals from a drive shaft position sensor 310. The spool 306 of the electrohydraulic valve 302 is oriented parallel to the axis 25 of the drive shaft 40, so as to move along a parallel axis. Note that the spool 306 of the electrohydraulic valve are located between drive shaft axis 25 the outward most position of the head of the piston 52 during each cycle. This location and orientation of the valve 302 further reduces the overall diameter of the hydraulic machine 300.

The first port 324 is coupled to all the control valves 302 for cylinder chambers 37 by the first annular passage 321 and the second annular passage 322 couples the second port 326 to all the control valves. In one position of the spool 306, the first annular passage 321 and thus first port 324 is in communication with the cylinder chamber 37. In a second position of the spool 306, the second annular passage 322 and second port 326 are in fluid communication with the cylinder chamber 37. The spool 306 has a center or third position in which the cylinder chamber 37 is closed from communication with both annular passage 321 and 322. Here too, the operation of the single three-position, three-way valve 302 replicates the action of the two check valves 33 and 34 in the first hydraulic machine 14 to pump fluid in one direction from the first port 324 to the second port 326. The operation of the three-position, three-way valve 302 can be reversed by the controller to pump fluid in the opposite direction from second port 326 to first port 324, thus enabling the hydraulic machine 300 to act as a bidirectional pump.

The three-position, three-way valve 302 also can be operated so that the hydraulic machine 200 functions as a motor. In that mode, the position of the spool 306 replicates the action of the first and second valves 202 and 204 described above for the hydraulic machine 200.

Employing a single three-position, three-way valve 302 for each cylinder chamber requires fewer electronic drivers in the controller than using two-position, two-way valves for each cylinder chamber. It also has the efficiency advantage of being able to have the piston cylinder blocked for decompression on the input shaft. This configuration also supports the use of the hydraulic machine selectively as a variable displacement pump or motor pumping fluid to either port and receiving high pressure oil from either port as a motor selectively as the application and controller dictate.

The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure. 

1. A hydraulic machine comprising: a cylinder block with an first port, a second port, and an exterior surface, and having a plurality of cylinders disposed radially in the cylinder block with openings through the exterior surface; a plurality of piston assemblies each slideably received in a different one of the plurality of cylinders; a plurality of valve arrangements each coupling one of the plurality of cylinders to the first and second ports; a drive shaft rotatably received in the cylinder block and having an eccentric cam for driving the plurality of piston assemblies within the plurality of cylinders; and a closing band engaging the exterior surface and closing the openings of the plurality of cylinders. 2-4. (canceled)
 5. The hydraulic machine as recited in claim 1 wherein each of the plurality of piston assemblies comprises a piston and a piston rod wherein the piston rod comprises a stem with a curved shoe through which force is transferred to the eccentric cam, wherein the curved shoe has a surface with an area that is greater than a largest cross sectional area of the stem.
 6. The hydraulic machine as recited in claim 5 further comprising a retaining ring extending around the drive shaft and engaging the curved shoe of every piston rod to constrain the curved shoe from moving away from the eccentric cam.
 7. The hydraulic machine as recited in claim 5 further comprising two retaining rings each extending around the drive shaft and engaging the curved shoe of every piston rod on opposite sides of the stem.
 8. The hydraulic machine as recited in claim 5 further comprising a cam bearing extending around the drive shaft and having an outer race with a plurality of rollers between the outer race and the eccentric cam, wherein the shoe of each piston rod has a surface that abuts the outer race of the bearing.
 9. The hydraulic machine as recited in claim 1 wherein the piston rod has an end with a partially spherical head that engages a partially spherical depression in the piston.
 10. The hydraulic machine as recited in claim 9 wherein piston has an interior cavity into which the partially spherical head of the piston rod is received.
 11. The hydraulic machine as recited in claim 10 further comprising a retainer within the interior cavity and holding the partially spherical head in engagement with the piston.
 12. The hydraulic machine as recited in claim 10 further comprising a spring within the interior cavity and engaging the partially spherical head of the piston rod and a slot in the piston.
 13. The hydraulic machine as recited in claim 1 wherein the piston and the piston rod are formed as a single piece, and the piston has a partially spherical circumferential surface that engages a wall of the cylinder.
 14. The hydraulic machine as recited in claim 1 wherein: the cylinder block includes a plurality of first bores in a first end surface and extending parallel to a longitudinal axis of the drive shaft, each of the plurality of first bores being in communication with the first port and opening into one of the plurality of cylinders, and the cylinder block also including a plurality of second bores in a second end surface and extending parallel to a longitudinal axis of the drive shaft, each of the plurality of second bores being in communication with the second port and opening into one of the plurality of cylinders; and the valve arrangement comprises a plurality of first valves, each located in one of the first bores, and a plurality of second valves, each located in one of the second bores.
 15. The hydraulic machine as recited in claim 14 further comprising a tubular sleeve within each one of the plurality of cylinders, wherein each tubular sleeve extends partially across openings of the first bore and the second bore into that one cylinder.
 16. The hydraulic machine as recited in claim 14 wherein each of the first valves and the second valves is an electrically operated bidirectional valve.
 17. The hydraulic machine as recited in claim 16 wherein each of the first valves and the second valves is a two-position, two-way valve.
 18. The hydraulic machine as recited in claim 1 wherein: the cylinder block includes a plurality of bores in a first end surface and extending parallel to a longitudinal axis of the drive shaft, each of the plurality of bores in fluid communication with the first port, the second port, and a different one of the plurality of cylinders; and the valve arrangement comprises a plurality of electrically operated valves, each located in one of the plurality of bores for selectively connecting the associated cylinder with the first and second ports.
 19. The hydraulic machine as recited in claim 18 wherein each of the plurality of electrically operated valves is a three-way valve.
 20. The hydraulic machine as recited in claim 1 wherein the drive shaft has a passage with an opening in a surface of the eccentric cam for conveying fluid for lubrication.
 21. The hydraulic machine as recited in claim 20 wherein the passage of the drive shaft is operably connected to receive fluid that enters the hydraulic machine through one of the first port and the second port.
 22. A hydraulic machine comprising: a cylinder block with an first port, a second port, and an exterior surface, and having a plurality of cylinders disposed radially in the cylinder block with openings through the exterior surface; a closing band engaging the exterior surface and closing the openings of the plurality of cylinders; a drive shaft rotatably received in the cylinder block and having an eccentric cam; a cam bearing extending around the drive shaft and having an outer race with a plurality of rollers between the outer race and the eccentric cam; a plurality of piston assemblies each slideably received in a different one of the plurality of cylinders and comprising a piston and a piston rod, wherein the piston rod comprises a stem with a curved shoe abutting the outer race; a retaining ring extending around the drive shaft and engaging the curved shoe of every piston rod to hold the curved shoe against the outer race; and a plurality of valve arrangements each selectively coupling one of the plurality of cylinders to the first and second ports. 23-26. (canceled)
 27. The hydraulic machine as recited in claim 22 wherein the valve arrangement comprises a plurality of first valves, each coupling one of the plurality of cylinders to the first port; and a plurality of second valves, each coupling one of the plurality of cylinders to the second port.
 28. The hydraulic machine as recited in claim 27 wherein each of the plurality of first valves and the plurality of second valves is an electrically operated bidirectional valve.
 29. The hydraulic machine as recited in claim 22 wherein the valve arrangement comprises a plurality of electrically operated, three-way valve, each selectively coupling one of the plurality of cylinders to the first and second ports. 30-36. (canceled)
 37. A hydraulic machine comprising: a cylinder block having a first port, a second port, an end surface, a side surface, and a plurality of cylinders disposed radially in the cylinder block with openings through the side surface, the cylinder block further including a plurality of bores in the first end surface and each bore in fluid communication with the first port, the second port, and one of the plurality of cylinders; a drive shaft having an eccentric cam and rotatably received in the cylinder block and projecting outward therefrom; a plurality of piston assemblies slideably received in a different one of the plurality of cylinders; and a plurality of valve arrangements, each located in one of the bores and selectively controlling fluid flow between the one of the plurality of cylinders and each of the first and second ports.
 38. The hydraulic machine as recited in claim 37 further comprising a closing band engaging the side surface and closing the openings of the plurality of cylinders.
 39. The hydraulic machine as recited in claim 37 wherein each of the plurality of valve arrangements is an electrically operated three-way valve.
 40. The hydraulic machine as recited in claim 37 wherein each of the plurality of valve arrangements is a three-position, three-way valve.
 41. The hydraulic machine as recited in claim 37 wherein each of the plurality of valve arrangements comprises: a plurality of first valves, each located in one of the bores and selectively controlling fluid flow between the first port and the one of the plurality of cylinders; and a plurality of second valves, each located in one of the second bores and selectively controlling fluid flow between the second port and the one of the plurality of cylinders.
 42. The hydraulic machine as recited in claim 37 wherein each of the plurality of piston assemblies comprises a piston, a piston rod having a stem with a curved shoe through which force is transferred to the eccentric cam, and a retaining ring extending around the drive shaft and engaging the curved shoe of every piston rod to constrain the curved shoe from moving away from the eccentric cam. 